The first line of the review: "The forums have been buzzing with wonder and excitement over the new Hifiman Sundara planar magnetic headphones." But have the forums really been buzzing about the Sundara? Not that I can tell.
The metal band is no longer squarish but more of an arc. Looks better, but one of the things I love about the HE560 is that, to adjust the comfort/clamping force, I can bend the metal band in the middle, and if I bend it too much I can always bend it back so that it hardly looks like it was tampered with. Wonder if it'll be the same with the new band. I'm sure it will bend, but will it bend back into place perfectly?
Sound-wise, it's nice that the Sundara has a better low end than the 400i, though according to the reviewer still not as good as the 560. But he also describes it as a thick-sounding headphone, both in the low and mid range. That doesn't sound like the 560 to me, to which Hifiman had been comparing the Sundara.
Soundstage looks like a big improvement over the 400i: "I was impressed by the sense of depth in the Hifiman Sundara. It is a bigger sense of depth than the 400i and felt on par with the HE-560. Additionally, the sense of width was really nice and expansive."
Bottom line according to the review: "The Hifiman Sundara planar magnetic headphones are certainly an upgrade from the HE-400i. And as a standalone headphone, there are some things I like, some things I don’t. I do think the HE-560 is better for most purposes, although it is more difficult to drive. However if you listen to music without drums, you may just prefer the thickness of the Hifiman Sundara."
Because the 560 came down in price to what the Sundara costs, I think most people will expect the Sundara to be an improvement on the 560. But I guess if it improves on the 400i, that should be good enough, especially once the 560 goes out of stock.